An eddy current probe uses an alternating current that flows through a wire coil and generates an oscillating magnetic field. If the probe and its magnetic field are brought close to a conductive material like a metal test piece, a circular flow of electrons known as an eddy current moves through the metal and generates its own magnetic field, which interacts with the coil and its field through mutual inductance.
Changes in metal thickness or defects like near-surface cracking interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field. This in turn affects the movement of electrons in the coil by varying the electrical impedance of the coil. The eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used by a trained operator to identify changes in the test piece.
One use of eddy current instruments is to examine cracking inside bolt holes, often with automated rotary scanners. The signals returned from the probe are displayed using an impedance plane plot that graphs coil resistance on the x-axis and inductive reactance on the y-axis. Variations in the plot correspond to variations in the test piece.
Conventionally the rotary scanners employ a “dual probe” or differential eddy current probe to scan the interior of the bolt holes. When the differential probe (two probes) passes a crack or a few cracks, the direct response of the differential probe shows on the display as a figure “8”, with the knot of the two circles representing the moment when the crack is right between the two coil sensors. The signals from a probe inserted into a test block with a standard crack flaw typically produce displays on an impedance plane as a “figure 8”. When using the instrument configured in this way, the analyst may accidentally pass over some flaws because they produce a display pattern very similar to the standard crack “figure 8” but flipped over in such a way that they appear very similar.
In calibrating the eddy current rotary scanner, it is a long known practice that eddy current field engineers look for “backwards 6” figures on the impedance plane display as an indication of a standard indication (the standard crack) or default status of the tube or bolt hole inspections. This calibration set up is often a preferred alternative to the “figure 8” as a standard indication display. When an abnormality exists, such as a crack or corrosion, the “backwards 6” changes in shape or direction to some degree. Although the raw detected signal response is presented as a “figure 8”, it is not desirable, since the abnormality often shows in the first region of the coordinate and the “figure 8” confuses the viewing of the abnormality. In existing practice, an Infinite Impulse Response (IIR) filter is used to shift the phase of frequency response for signals lower that certain frequency.
One problem with this type of prior art design is that it causes inspectors in the field to have to deal with a “backwards 6” drastically changing in size, or changing into a “figure 8” when the revolutions per minute (RPM) speed of the rotary scanner changes. When the frequency decreases, the size of the “backwards 6” becomes smaller; when the frequency increases, the size of the “backwards 6” becomes bigger; and when the frequency increases beyond the range of the IIR filter, the “figure 8” displays.
Considering the background information above, a solution that by design provides a steady display of an eddy current inspection result without the need to readjust the amplitude and frequency settings of the instrument would be of great economic value. Rotary scanner such as those for bolt hole inspections would take place in less time and with more effectiveness. The task of the inspection would be visually much more pleasant to handle.